Copy payload data when inserting packets into video_coding::PacketBuffer.
Copy payload data when inserting packets into video_coding::PacketBuffer.

The payload pointed to by |dataPtr| is volatile and needs to be copied
to its own buffer.

BUG=webrtc:5514
R=brandtr@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2302763002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14321}

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